Stereoisomerism in Octahedral Complexes

The simplest type of stereoisomerism for hexacovalent systems occurs in cases where four of the six substituents on a complex are the same. The remaining two, which may or may not be different from each other, may occupy either cis or irans positions (Fig. 22-14). Both of these complexes are superimposable on their mirror images and cannot be separated into enantiomorphic forms. This can be checked by imagining the complexes to be reflected in a mirror (as was done in Fig. 22-7), or it may be noted that both complexes have at least one plane of symmetry (that is, that there is at least one way in which a plane can bisect the complex into two mirror-image halves). The presence of a plane of symmetry is a convenient indication of the nonresolvability of a complex. The apparent absence of such a plane, however, should be regarded with reserve, for without considerable experience or a good sense of spatial 

Note that the existence of two and only two isomers of hexacovalent complexes of the type MX2Y4 verifies the octahedral structure of such complexes in preference to structures derived from the hexagon (a) or trigonal prism (b) (Fig. 22-15) (Exercise 7). Sometimes, isolating the cis 

22-19 pictures a typical chelating agent of this type and a complex formed from it. From the simplified picture, one may see that the complex is nonsuperimposable on its mirror image. 

SPIN-PAIRED AND SPIN-FREE COMPLEXES LIGAND FIELD EFFECTS 

Up until the early IQSG s, it was customary to think of the bonding in metal complexes ifi much the same terms as that in simpler compounds of the nonmetals. Thus the Co—N bonds in Co(NH )e3 were said to involve d2sp hybridization (like the bonds in SF ), whereas the Zn—O 

---